I useded the SysInternals (err, Microsoft) Process Monitor Tool and watched for keys found/not found during the failed install. This found a few keys in HKCR\Installer\UpgradeCodes that were being found early in the setup100.exe process.

Reverse it. I think I had 615fe700, but it was late and I was tired. It might have been 16f57e00. Anyways, you will see some keys under HKCR\Installer\UpgradeCodes. There are actually 2-3 places in the registry searched for each key I’ve been killing all three of them for each key – there are about 7-10 keys. The registry section looks like this:

The hex digits you have in the error will correspond to the right hand side of this picture. The key I’ve been deleting is it’s parent, which is the key being opened in the “key found”/”key not found” stuff in the process monitor log.

Now the installer gets past this and tries to install the engine and then fails, but I will call this progress ;).

Notice – I had deleted all of my physical files for SQL Server from the machine, so killing the registry keys seemed like a reasonable next step. I can’t promise you it’s a good idea since I don’t have things working yet, but I hope this helps the many of you who mailed me and found me via search engines.